NOx generated from stationary sources, including industrial boilers, gas turbines, combined cycle power plants, waste incinerator, vessel engines and petrochemical plants, is a major cause of reduced visibility, the greenhouse effect (N2O) and acid rain, is known to be the main cause of air pollution, and binds to oxygen in the presence of UV light to cause photochemical smog. Technology for removing such NOx can be broadly divided into primary methods, known as fuel de-nitrification and combustion modification, and a secondary method known as fuel gas treatment. In the case of fuel de-nitrification, among these methods, HDN (hydro-denitrification) is the most general, and describes a process whereby organic nitrogen contained mainly in fuel is converted into ammonia by reaction with hydrogen, so that the emission of pollutants can be inhibited. However, it requires large facility investment and cost, and the effective reduction of NOx in fuel is possible, whereas the effect of reducing thermal NOx cannot be expected. Also, methods of employing modified combustion conditions or combustion methods, for example, flue gas recirculation and air register adjustment, are currently used in practice to a considerable extent, but have a limitation on NOx reduction efficiency (40-70% of NOx emission), and thus have difficulty satisfying emission regulations, which are becoming stricter.
In view thereof, post-combustion treatment, i.e., flue gas treatment, is used for the synthetic reduction of NOx. The flue gas treatment processes are generally divided into wet treatment processes and dry treatment processes. The wet treatment processes require high facility investment and operating costs due to the low solubility of NO, and have a serious problem associated with wastewater treatment. For this reason, a lot of research aimed at increasing the NOx removal efficiency to realize the practical use of the dry treatment methods has been conducted. In connection with this, selective reduction processes, of selectively removing only NOx using a reducing agent, occupy the majority of the dry treatment processes and are divided, according to whether a catalyst is used, into selective non-catalytic reduction (SNCR) and selective catalytic reduction (SCR).
The selective non-catalytic reduction process is a technique in which a reducing agent is injected into an area having a combustion gas temperature of 850-1100° C. in a furnace without using any catalyst, so that NOx is selectively reduced into nitrogen and water vapor by reaction with the reducing agent. Parameters having a decisive effect on the selective non-catalytic reduction process include the injected amount of the reducing agent, the reaction temperature, the residence time of the reducing agent, and the mixing of the reducing agent with combustion gas, and an increase in the amount of use of the reducing agent leads to an increase in NOx reduction efficiency. However, the selective non-catalytic reduction method has NOx removal efficiency about 30-50% lower than that of the SCR process and incurs high process maintenance costs due to the excessive use of the reducing agent and the high temperature operating conditions. Also, it can cause adhesion/corrosion problems in a downstream heat exchanger due to the production of salts, such as ammonium bisulfate (NH4HSO4) and ammonium sulfate (NH4)2SO4), which arise from unreacted ammonia.
For this reason, the selective catalytic reduction (SCR) process, which can show high denitrification efficiency by increasing the reaction rate between a reducing agent and NOx in a lower temperature range using a catalyst, is recognized as the most advanced denitrification process. As shown in reaction equations 1-4 below, the SCR process is characterized in that NOx is removed in the form of harmless nitrogen and water vapor by mixing NOx with ammonia as a reducing agent at an operating temperature of 250-450° C. and passing the mixture through a catalyst layer. However, in fact, NOx is removed according to the reaction equations 3 and 4, because flue gas contains oxygen.6NO+4NH3→5N2+6H2O  [Reaction Equation 1]6NO2+8NH3→7N2+12H2O  [Reaction Equation 2]4NO+4NH3+O2→4N2+6H2O  [Reaction Equation 3]2NO2+4NH3+O2→3N2+6H2O  [Reaction Equation 4]
However, in the above process, ammonia to be used as the reducing agent reacts with oxygen to produce nitrogen and NOx, as shown in reaction equations 5-8 below.4NH3+3O2→2N2+6H2O  [Reaction Equation 5]4NH3+4O2→2N2O+6H2O  [Reaction Equation 6]4NH3+5O2→4NO+6H2O  [Reaction Equation 7]4NH3+7O2→4NO2+6H2O  [Reaction Equation 8]
In general, NH3 oxidation actively occurs as the reaction temperature increases to high temperatures, and it occurs in competition with NOx reduction, and thus the conversion rate of NOx changes depending on the temperature. If water is not included, the reaction according to the reaction equation 6 will not substantially occur, NOx will be generated specifically according to the reaction equations 7 and 8, and thus the reactions according to the reaction equations 7 and 8 should be inhibited. However, the rates of such reactions will increase with an increase in temperature. For this reason, in order to prevent side reactions from occurring at high temperatures, there is an urgent need to develop a catalyst having excellent selective NOx reduction performance even at low temperatures. Meanwhile, flue gas generally contains water and sulfur oxides, which produce salts on the catalyst, thus reducing the activity of the catalyst. Reactions that are the main cause of this catalyst poisoning occur according to reaction equations 9-12 below.2NH3+H2O+2NO2→NH4NO3+NH4NO2  [Reaction Equation 9]2SO2+O2→2SO3  [Reaction Equation 10]NH3+SO3+H2O→NH4HSO4  [Reaction Equation 11]SO3+H2O→H2SO4  [Reaction Equation 12]
The reaction equation 9 is a reaction in which NO2 reacts with NH3 to form ammonium nitrate, and it is known that such ammonium nitrate is decomposed at a temperature higher than 150° C., and thus does not cause catalyst poisoning. However, in actual processes, ammonia is injected at a temperature higher than 150° C., and thus catalyst poisoning is induced, because SO3 produced according to the reaction equation 10 forms sulfate according to the reaction equation 11, and the sulfate remains on the catalyst surface without being decomposed. Also, sulfuric acid is produced according to the reaction equation 12, and can be a cause of corrosion of the catalytic layer and downstream facilities. For this reason, there is a need to develop a low-temperature, high-activity denitrification catalyst which can suppress the production of sulfate and sulfuric acid according to the reaction equations 11 and 12.
In general, as catalysts that are used in the selective catalytic reduction technique, various catalysts, including noble or transition metal catalysts and basic catalysts, have been proposed. Recent SCR catalysts have been mostly studied with respect to vanadium, and it is known that the use of V2O5 loaded on a TiO2 support shows the most excellent effect on selective catalytic reduction.
The prior art relating to such V2O5 catalysts is as follows.
U.S. Pat. No. 4,152,296 discloses a process for preparing a catalyst for the denitrification of flue gas, which comprises impregnating a TiO2 support with at least 1% by weight (preferably 0.35-1.35% by weight) of a vanadium atom based on the weight of said support of vanadium sulfates (VSO4), vanadyl sulfates (VOSO4) and a mixture thereof and then reacting a mixed gas of ammonia and inert gas with the impregnated support at a temperature of 300-520° C. The prepared catalyst has a pore volume of 0.3-0.45 mL/g and a specific surface area of 20-50 m2/g.
U.S. Pat. No. 4,929,586 discloses a catalyst for the removal of NOx, in which a catalytic component selected from the group consisting of V2O5, MoO3, WO3, Fe2O3, CuSO4, VOSO4, SnO2, Mn2O3, and Mn3O4 is supported on a titania (TiO2) support having an anatase crystal structure. Also, it discloses that the catalyst has an NOx conversion rate of about 90% at a temperatures around 350° C. According to the disclosure of said patent, TiO2, used as the support, has a total porosity of up to 0.80 cc/cc, which is made up of a micropore porosity (comprising pores having a pore diameter 600 Å or less) of 0.05 to 0.5 cc/cc and a macroporosity (comprising pores having diameters greater than 600 Å) of 0.05 to 0.5 cc/cc.
U.S. Pat. No. 5,045,516 discloses a process for preparing a catalyst for reducing NOx, in which molybdenum trioxide and less than 10 wt % of vanadium pentoxide are supported on TiO2. In said patent, in order to prevent the catalyst from being inactivated by catalyst poisons, such as arsenic compounds in flue gas, the contents of calcium and iron in the TiO2 composition are limited to less than 500 ppm and 100 ppm, respectively, said TiO2 being present in a proportion of over 60% as the anastase type structure, and having a mean particle size of 10 to 100 nm, a mean pore radius of 10 to 30 nm and a BET surface of 10 to 80 m2/g.
U.S. Pat. No. 6,054,408 discloses a method for preparing a catalyst for reducing NOx, in which 0.01-5 wt % of molybdenum trioxide and 0.01-5 wt % of vanadium pentoxide are supported on anatase-type TiO2. In the case of TiO2, used as the support in said patent, the proportion of rutile type in the crystal structure is limited to less than 5%, the content of sodium, potassium and iron is limited to less than 500 ppm, and the content of phosphorus is limited to less than 0.5%.
U.S. Pat. No. 4,952,548 discloses a catalyst for removing NOx, in which the atomic ratio of Ti:Mo or W:V is 80-96.5:3-15:0.5-5. In particular, in order to prevent poisoning caused by the adsorption of heavy metals on the TiO2 surface, the size of the crystallite of the titanium oxide according to Sherrer's equation is in the range of 185-300 Å in the direction of a (101) phase.
U.S. Pat. No. 4,916,107 discloses a catalyst for removing NOx, comprising: titanium oxide, tungsten oxide; and at least one selected from the group consisting of vanadium, iron, niobium and molybdenum. Specifically, TiO2 used as the support is mainly present in the form of anatase, has an average primary particle size of 30 nm, a loss on drying of 1.5 wt % and a loss on ignition of 2 wt %, and comprises 99.5 wt % TiO2, 0.3 wt % Al2O3, 0.2 wt % SiO2, 0.01 wt % Fe2O3 and 0.3 wt % HCl.
The above-described patents specify only the physical properties of TiO2, used as the support, and the content of vanadium in the preparation of NOx removal catalysts, and comprise adding a secondary metal in order to increase reaction activity and resistance, but do not mention the effect of changes in the properties of catalysts on selective catalytic reduction. Also, an example where a mechanochemical method for high energy transfer is applied for the preparation of a selective reduction catalyst, as disclosed in the present invention, cannot be seen in the above patents.
Till now, techniques of imparting high denitrification efficiency and SO2 resistance by adding tungsten, molybdenum and the like to vanadium/titania-based catalysts for flue gas denitrification have been somewhat commonly used. Such techniques show relatively high denitrification efficiency at a temperature of about 350° C., but show low denitrification efficiency at a temperature lower than 260° C., and thus impose a limitation on the installation of the SCR system. Generally, the SCR process in power plants is provided in upstream or downstream of a desulfurization system, and when it is provided in upstream of the desulfurization system, abrasion problems, caused by catalytic fatigue and dust, and life-cycle reduction problems, caused by sulfur dioxide poisoning, will arise. In consideration of such problems, the SCR process is generally provided after the desulfurization facilities. In this case, the temperature of flue gas rapidly decreases after desulfurization, because the desulfurization process is conducted as a wet process. For this reason, an additional heat source is required to attain optimal efficiency, and thus the use of large amounts of energy is inevitable. Also, the production of salts, such as NH4NO3 and NH4HSO4, which are generated in a low-temperature range, has problems in that the salts corrode systems, shorten the life cycle of catalysts and reduce denitrification efficiency. To solve the above-described problems, various attempts have been made, but have not yet reached a satisfactory step.
In connection with this, Korean Patent Application No. 2002-17168, filed in the name of the present applicant, discloses a catalyst for removing nitrogen oxides and/or dioxin, in which vanadium oxides are supported on a titania support, capable of providing lattice oxygen, in an amount of 0.3-10 wt % based on the weight of the titania, and the non-stoichiometric ratio of the vanadium oxides, defined as the molar ratio of VOx (x<2.5)/V2O5, is more than 2.0, said VOx containing V2O3 as a main component. According to the disclosure of this prior art, titania, capable of providing lattice oxygen to vanadium is used in the preparation of the catalyst to form VOX (x<2.5, mainly vanadium trioxide) in addition to V2O5 as catalytic component vanadium oxides, and the non-stoichiometric ratio of the vanadium oxides is set at more than 0.2, such that the lattice oxygen on the catalyst surface can easily participate in selective catalytic reduction due to electron migration between the metal oxides of the catalyst and reactants during the reaction of ammonia with nitrogen oxides. This technology is a technology of using a specific support in the preparation of a catalyst to form non-stoichiometric vanadium oxides, but there is still a limitation on the use of support. Thus, in order to apply optimal denitrification systems in various fields, there is a need to develop a high-activity denitrification catalyst, which can maintain high activity even in a low temperature range and is particularly suitable for commercial purposes, unlike the prior commercial vanadium/titania-based catalysts.